It is known to prepare high-molecular weight, linear, soluble, thermoplastic aromatic polycarbonates by reaction of bisphenols with polycarbonate-forming derivatives of carbonic acid under polycarbonate-forming reaction conditions, say according to the processes of German Patent Specifications Nos. 971,790; 971,777; 959,497; 1,007,996; 1,031,512 and 1,046,311. In contrast to most other plastics, melts of linear polycarbonates exhibit Newtonian flow in the range of low rates of deformation (=rates of shear), and non-Newtonian flow in the range of high rates of deformation. For extrusion processes, it is advantageous to have values of the apparent melt viscosity which are as high as possible. On the other hand, for processing by injection molding, it is desirable that the apparent melt viscosity should have values which are as low as possible. In spite of the non-Newtonian flow of linear polycarbonates in the range of high rates of deformation, such as occur during processing by injection molding, the accompanying lowering of the apparent melt viscosity is too slight--that is to say the apparent melt viscosity is still too high--for linear polycarbonates of high apparent melt viscosity to be employed for injection molding. For this reason, linear polycarbonates with low apparent melt viscosity are normally suitable for processing by injection molding and those with high apparent melt viscosity are normally suitable for extrusion processes. In other words, in general, it is not possible to employ one and the same linear polycarbonate, particularly if the apparent melt viscosity has a high value, both for processing by extrusion and for processing by injection molding.
Attempts have also been made to modify linear polycarbonates of high solution viscosity, through incorporation of functional bisphenols, in such a way that they show non-Newtonian flow in the range of very low rates of deformation (processing by extrusion), and hence exhibit high shape retention of the extruded ribbon. Such products are particularly suitable for the manufacture of hollow articles of large volume in accordance with the extrusion-blowing process (compare DT-OS (German Published Specification) No. 1,595,762).
Polyfunctional, especially trifunctional and tetrafunctional, phenols (compare DT-OS (German Published Specification) 1,595,641, DT-OS (German Published Specification) No. 1,570,533, DT-OS (German Published Specification) No. 2,039,536, U.S. Pat. No. 3,541,049, DT-OS (German Published Specification No. 2,113,347, U.S. Pat. No. 3,799,953 and DT-OS (German Published Specification) No. 2,254,918) or bisphenols having an additional radical capable of undergoing condensation such as, for example, the carboxyl group (compare DT-OS (German Published Specification) No. 1,595,762) are known as branching agents for aromatic polycarbonates.
In part, polyhydric phenols of functionality greater than two are only obtainable via expensive purification processes and at times show considerable differences in respect of the reactivity of the phenolic hydroxyl groups. A reduced reactivity of individual OH groups, especially of OH groups in middle positions, towards carbonate-forming compounds does not ensure complete incorporation of the compounds, so that greatly fluctuating degrees of branching must be expected. The free phenolic hydroxyl groups which are still present, because of the incomplete conversion, can lead to undesired discolorations of the material on repeated extrusion. For example, in the blow-molding process re-extrusions must be expected particularly frequently, since after trimming the hollow articles to the desired shape up to 20% of excess material arises, which must be added back to the next process cycle if it is desired to operate economically. Hence, it is only possible to incorporate those branching agents which even after repeated extrusion lead to at most very slight discolorations of the polycarbonate.
In the case of branching via cyanuric chloride, as described in U.S. Pat. No. 3,541,049, the functional groups of the branching agent again show a marked gradation in reactivity. Furthermore, in the case of polycarbonate branched via cyanuric chloride, the material shows yellowing phenomena after exposure to light.
Again, in the case of the polyfunctional phenols described in DT-OS (German Published Specification) No. 1,595,762 especially bisphenol-acids, the incorporation achieved is at times only 75%.
The ortho-esters described in DT-OS (German Published Specification) No. 2,254,918 and U.S. Pat. No. 3,897,392 have the disadvantage that in the solution condensation process the branched polycarbonates are not obtained directly; instead, the ortho-esters must first be converted into sufficiently reactive branching agent precondensates by an intermediate reaction with aromatic dihydroxy compounds.
Further, it was known from U.S. Patent No. 3,256,242 to manufacture aromatic polyesters containing chlorine by condensation of chloroisophthalic acid dichloride with bis-(4-hydroxyphenyl)-oxindole in equimolar ratios at 150.degree.-250.degree. C. This gave solely linear polyesters but no crosslinked or branched products and no polyester-imides, as can be seen from the structural formulae and examples in the above specification. In other words, under the conditions of the condensation disclosed in the above specification, the formation of linear ester bonds is preferred, since otherwise the amide group of the oxindole would also, at least partially, have to undergo a condensation, even if the reactants are employed in the equimolar ratio instead of employing 1.5 mols of dicarboxylic acid per mol of bis-(4-hydroxyphenyl)-oxindole.
It was the object of the present invention to discover a new branching agent for linear, in particular aromatic, polycarbonates, which is practically completely incorporated into the polycarbonate molecule in the processes of phase boundary condensation, solution condensation and melt condensation customary for the preparation of polycarbonates, so that discolorations largely do not occur, even on repeated extrusion. Furthermore, the branching agent, even if employed in small amounts, should impart a pronounced structural viscosity behavior to the branched polycarbonates obtained so that a grade of polycarbonate is equally suitable for processing by extrusion and processing by injection molding, that is to say the branched polycarbonates should, in the range of rates of deformation which are used for processing by extrusion and processing by injection molding, exhibit a marked dependence of the apparent melt viscosity on the particular rate of deformation. At the same time, the apparent melt viscosity should be as high as possible at low rates of deformation (extrusion) and as low as possible at high rates of deformation (processing by injection molding).
The object has been achieved by employing 3,3-bis-(4-hydroxyaryl)-oxindoles (hereafter also referred to as isatin-bisphenols) for the manufacture of high-molecular weight, thermoplastic, soluble and branched polycarbonates.